1. Field of the Invention
The invention relates to a method and a device for detecting simultaneous multiple transmission in communications systems with amplitude-modulated signals, in particular, aeronautical radio equipment in air-traffic control (ATC).
2. Related Technology
With regard to the prior art, reference is made to U.S. Pat. No. 6,823,025 B2, from which a detector for amplitude modulation is known. However, the detection of a double transmission is not possible in this context.
The constantly growing volume of traffic in civil aviation increases the danger of simultaneous double transmissions on the same channel. Especially in busy airports, it can happen that two pilots establish contact with the air-traffic controller simultaneously, without the air-traffic controller or the pilots noticing this hazardous situation. The air-traffic controller hears only the stronger signal and acknowledges the radio communication. By contrast, the unheard pilot thinks that the response from the air-traffic controller applies to him.
The frequently used AM-DSB modulation method (amplitude modulation—two side bands) behaves in a linear manner with regard to multiple reception, so that the weaker received signal (referred to below as the secondary signal) is demodulated in a proportionally quieter manner. Because of the limited signal to noise ratio and background noises in the cockpit, a signal relatively weaker by 20 to 30 dB can hardly be detected.
So long as the quartz precision of the aeronautical radio equipment actually exhausts the range from +/−5 ppm originally specified by the ICAO (International Civil Aviation Organization), a readily-audible whistling tone with the differential frequency of the two aeronautical radio devices occurs. Contemporary aeronautical radio equipment with modern reference oscillators provide very good frequency accuracy with deviations of less than 1 ppm, so that the differential frequency is generally disposed below 100 Hz and is filtered out in the audio filter of the stationary receiver equipment. The simultaneous transmission of two or more signals on the same channel is not generally detected in aeronautical radio equipment.